You do not have sufficient privileges to delete OU or this object is protected from accidental deletion

Windows Server 2008 has this new feature to protect the Organizational Units from accidental deletion.

If you wish to delete an OU which had 'Protect container from accidental deletion', open Active Directory Users and Computers and select 'Advanced Features' under the 'View' menu

Now right click the OU you wish to delete and select 'properties'. Under the 'Object' tab uncheck 'protect object from accidental deletion'